The present invention relates generally to clutch mechanisms for turbine engines, turbopumps and the like, and more particularly to a bearing engagement mechanism in a bidirectional conical carbon-carbon clutch structure.
Standard lubrication systems are being pushed to the limit in current turbine engines. Conventional rolling element bearings and liquid lubricants become unusable as the thrust-to-weight ratio for engines increase. One way to increase the temperature and speed capability of a bearing sump is by using magnetic bearings. To meet thrust-to-weight goals of advanced turbine engines, magnetic bearings are generally excessively large and heavy for handling short duration, high load maneuver conditions and auxiliary conventional rolling element bearings must be used to support the shaft.
Conventional rolling element bearings used as auxiliaries must have a clearance between the shaft and the inner race (0.005-0.010-0.020 inch diametral clearance) which is less than the magnetic bearing clearance. However, in the event of touchdown on the auxiliary bearings, this gap usually produces dynamic instability for the rotating shaft. An example of shaft instability is a condition known as backward whirl where the shaft bounces inside the tolerance of the inner race in a direction opposite to rolling. Additionally, when the auxiliary bearing is engaged the inner race tries to attain shaft speed instantaneously, resulting in skidding damage to the bearing. Simple sleeve bearings can be used to bring the shaft to a stop after a magnetic failure, but sleeve bearings are not adequate for a shared load condition because they experience high wear unless they are lubricated under a fully flooded condition of up to two gallons of lubricant per minute. This would necessitate a lubricant reservoir, pump, and cooler.
Continuously engaged rolling element bearings with softly mounted races have the problem of limited life since high temperature experimental lubrication schemes with marginal lubrication capability must be used. These high temperature experimental lubrication schemes produce high wear and bearing life is typically less than 30 hours.
Magnetically levitated rotors for aerospace turbine engines require auxiliary bearings for shaft support in case of magnetic bearing failure or overload. For engine applications a magnetic bearing system cannot be sized to handle full maneuver loads because it becomes unrealistically large and heavy. Thus, auxiliary bearings are required to handle loads above the capacity of the magnetic bearings. To use conventional rolling element bearings on an as-needed basis, means must be used to close the tolerance from a disengaged to an engaged status while centering the shaft. Closing the clearance around the shaft prevents backward whirl. Centering the shaft minimizes rotating unbalance. A relatively gradual acceleration of the bearing is required to avoid skidding damage and inertial welding. Ideally the rolling element bearing is brought up to speed quickly, in the order of a few seconds rather than almost instantaneously. The present invention allows a gradual startup of the auxiliary bearing through the use of a slip surface or clutch consisting of one or a pair of carbon-carbon clutch plates or rings. The clutching action of the present invention allows the bearing elements to come up to shaft speed gradually during engagement, thereby minimizing skidding damage. The closed clearance avoids backward whirl. The centering minimizes shaft unbalance.
The invention is a solution that uses a rolling element bearing and provides a gradual engagement to minimize skidding damage of the bearing, provides closed clearance to avoid backward whirl, and provides shaft centering to minimize rotating unbalanced loads.
An auxiliary bearing engagement mechanism using a carbon-carbon clutch enables the use of conventional rolling element bearings as auxiliary bearings for as-needed use in magnetically supported rotors. Rolling element bearings must have a gradual engagement and provide shaft centering. Conventional rolling element bearings with 0.005-0.010-0.020 inch diametral clearance between the shaft and bearings have been shown to be dynamically unstable with skidding damage, inertial welding, and a catastrophic backward whirl condition when used for auxiliary support.
It is therefore a principal object of the invention to provide an improved clutch mechanism.
It is a further object of the invention to provide an improved carbon-carbon clutch mechanism having particular utility within turbine engines, turbo pumps and the like.
It is another object of the invention to provide a novel bearing engagement mechanism in a bi-directional conical carbon-carbon clutch structure.
It is another object of the invention to allow a gradual startup of a conventional rolling element auxiliary bearing through the use of a slip surface or clutch consisting of one or a pair of carbon-carbon clutch plates or rings.
It is another object of the invention to allow conventional rolling bearing elements to come up to shaft speed while preventing skidding damage and rotor backward whirl.
These and other objects of the invention will become apparent in the detailed description of representative embodiments.
In accordance with the foregoing objects of the invention, a bearing engagement mechanism for a bi-directional conical carbon-carbon clutch for turbine engines, turbo pumps and the like is provided which includes a carbon-carbon clutch bearing engagement mechanism for the inner race of a rolling element bearing. The invention comprises a set of wave springs for axially urging the clutch into a non-engaged position, and guide means for guiding the clutch into an engaged position. The clutch includes a pair of conical carbon-carbon rings mounted in movable clutch ring housings. The guide means comprises opposing, contiguous pairs of non-parallel ball raceways and a ball positioned in both pairs of the ball raceways whereby relative rotation of the ball raceway pairs in opposite directions results in axial movement of the clutch ring housings in opposing axial directions. The carbon-carbon clutch provides a gradual engagement to minimize skidding damage of the bearing and provides shaft centering to eliminate the initial tolerance required to allow beneficial operation of the magnetic bearing while avoiding the backward whirl phenomenon.